The present invention relates to a projection system, more particularly to a system known as a back-projector. The back-projectors currently available on the market are systems which use CRT-based projectors or liquid crystal projectors as projector. However, the present invention can be applied to all types of projection systems.
A back-projector of known type comprises a projector emitting a light beam, a screen and means for sending the light beam to the screen. Currently, the means making it possible to send the light beam to the screen consist of plane mirrors arranged in such a way as to reflect the entire light beam over the whole surface of the screen while limiting the overall thickness of the various elements which have to be inserted into a housing of limited thickness. When a system of mirrors such as mentioned above is used, it is difficult to produce a wide screen, especially a 46-inch screen, since in this case the thickness of the projector remains too great.
A back-projector comprising means making it possible to fold the beam output by the projection objective in order to reduce the depth has therefore been proposed in European Patent Application No. 96402512.6 filed in the name of the Applicant.
As represented in FIG. 1, the projection system described in this patent application includes a projector 1 which can, for example, be a liquid crystal projector emitting a light beam 2 which is already linearly polarised. This beam 2 is sent to a quarter-wave plate 3 in such a way as to produce circularly polarised light. This circularly polarised light is transmitted through a cholesteric filter 4. To do this, the light is right-circularly polarised if cholesteric filters which reflect left-circularly polarised light are used. The beam transmitted through the filter 4 is then reflected off the plane mirror 5. During this reflection, the beam undergoes a phase shift of .pi., and it is then left-circularly polarised. The beam leaving the plane mirror 5 then encounters the cholesteric filter 4. Since the cholesteric filter reflects all left-polarised light, the light issuing from the mirror is returned to the mirror 5 where it undergoes a new phase shift of .pi.. The reflection off the plane mirror therefore converts left-polarised light into right-polarised light, which is then transmitted in full by the filter 4 to the screen, not represented.
In the system described in this patent application, three reflections take place, two off the plane mirror and one off the cholesteric filter, as represented clearly in FIG. 1. In European Patent Application No. 96402512.6, use is made of cholesteric filters or more generally polarisation filters which reflect light circularly polarised in a second sense opposite to the first sense, to obtain a projection system of small thickness. Now, the cholesteric filters currently available on the market have a number of constraints, especially as regards angle of incidence.
Now, reflective linear polarising filters have recently appeared on the market. These filters transmit linearly polarised light with little loss and reflect light polarised perpendicularly to the transmitted direction. However, in this case, a straightforward reflection off a mirror is not sufficient to reverse the state of polarisation. It is necessary to pass via a quasi-circular state in order for a mirror to give the polarisation from one linear state to another. The purpose of the present invention is to propose a projection system of the type described above, according to a structure which avoids stray reflections.